Conventionally, a so-called semi-finished glass product that can be processed further is manufactured. Such a semi-finished product is referred to as a “gob”, for example. A gob may e.g. have a substantially frustoconical shape. For further processing, a gob may undergo blank pressing, and a lens, a bottle or another glass object can be produced therefrom. For manufacturing reasons, outer surfaces of a gob may be substantially blank, transparent and bright. Conventionally, the outer surfaces are corrugated though. Particularly, a peripheral surface of the gob may be very corrugated.
If the gobs are further processed to form optical lenses for example, the demands on the gobs with respect to stains, bubbles, stones, and striae are very high. For this reason, the gobs are usually sorted manually and are systematically resorted if the number of critical blemishes and complaints increases. However, due to the high number of pieces involved in manufacturing and the difficult recognizability of defects, such as bubbles up to 0.2 mm and/or stones up to 0.05 mm, a visual inspection is limited.
Further difficulties in the visual inspection result e.g. from the distortion of the image of the gob and the defects contained therein, which difficulties are caused by the corrugation of the surface. In order to avoid the distortion, gobs with an almost identical refractive index are conventionally tested by immersion in liquid. However, owing to the required gob cleaning and drying, the liquid immersion is not practicable for testing several thousand gobs per day.